1. Field of the Invention
This invention relates to imaging using optical code reading devices. In particular, this invention relates to a system and method for reading and decoding optical codes using multiple color illumination.
2. Description of the Prior Art
As industry has continued to refine and improve production techniques and procedures, corresponding requirements have been levied for placing identifying data related markings upon components of manufactured assemblies. These markings enable tracking of, for example, the historical stages of a product's manufacture. Further, these markings enable components of complex machinery, such as automobiles and the like, to be identified, for example, during manufacture by quality control personnel, during the course of an investigation by governmental authorities, etc.
A variety of product marking approaches has been utilized in industry. For example, paper tags or labels carrying UPC codes are typically applied to components in the course of a product's assembly. This method provides identifying codes that are highly readable by prior art optical code readers. However, for many applications, such tags or labels may be lost, damaged or altered, thereby, rendering the coding useless. This shortcoming is well addressed by Direct Part Marking (DPM) and consequently DPM has gained widespread acceptance in industrial applications by providing durable markings, or code symbols, capable of being placed on a wide variety of surfaces.
DPM is the technique of directly imprinting or etching product and component surfaces with codes corresponding to a plurality of symbologies, and, in particular, high-density 2-D code symbologies, such as Data Matrix and QR Code. However, as a result of being etched directly onto a wide variety of material surfaces and textures, such as reflective or dark surfaces, properly illuminating, reading (and subsequently decoding) DPM codes is typically not possible with conventional code reading systems.
Readability of an imaged optical code is most often determined by contrast between the code symbol and its background surface. Optical codes, such as UPC symbols, printed on labels provide high contrast through selection of background and foreground colors—usually black geometrical shapes or foreground on a white background. The DPM method of marking objects with DPM codes does not have the luxury of selecting the background color or even a foreground color for the symbol markings, especially in the case of etching and dot peening (indention marking). Therefore, DPM codes often have low and inconsistent contrast resulting in low code readability, and subsequently, many misreads or no reads.
Accordingly, proper lighting is often necessary for reading and decoding DPM codes. Further, the precise angle of the originating light source, with respect to the DPM codes, needs to be controlled in order for the variations in surface texture created by the marking method to reflect the light with different intensities. Surface background conditions that are reflective, or yield very little visible symbol contrast to the naked eye under general ambient lighting, can become highly visible when illuminated by a light source at a given angle from the surface.
Conventional direct mark imaging systems and imagers use directional illumination to obtain contrast in dot-peened and laser-etched DPM code symbols. These systems and images include several banks of LEDs that are switched on at different times to illuminate and image a DPM code from various directions with the same color. The image with highest contrast is then selected for decoding.
As shown by FIGS. 1a–1c, each of the three images of FIG. 1 illustrate a DPM code being illuminated from three respective illumination directions using the same colored illumination. The contrast of the DPM code varies with each of the illumination directions. Successful decoding of the DPM code depends on properly illuminating the DPM code from at least one of the three different illumination directions to get at least one readable and decodable contrast of the DPM code.
Similar to the set up shown by FIGS. 1a–1c, prior art DPM or other imagers utilize monochrome detector arrays for acquiring a DPM code in three separate imaging frames. Each frame represents the DPM code as illuminated from a different illumination direction. Each image requires an acquisition time of about 30 ms, thereby requiring, at a minimum, 90 ms for data acquisition and decoding of DPM codes by prior art imagers.
The long acquisition time is mainly due to prior art imagers using a light source capable of illuminating using a single color or relying on ambient light for illumination. Hence, prior art imagers can only image a DPM code from one illumination direction at a time. Therefore, typically, after images corresponding to each illumination direction are acquired, can a prior art imager proceed with the processing of the acquired images to determine likelihood of decodability for each image.